Efficient use of water resources is demanded due to industrial developments or population growth in recent years. For that purpose, reuse of wastewater such as industrial wastewater is very important. In order to attain the reuse, it is required to separate foreign substances from wastewater, i.e., to purify water. As a method of separating foreign substances from a liquid, various kinds of methods are known. The methods include removal of suspended substances by means of membrane separation, centrifugal separation, activated carbon adsorption, ozonization, and condensation, etc., for example. Such a method allows it to remove chemical substances greatly influential to environments, e.g., phosphorus contained in water, nitrogen and oil/clay dispersed in water, etc. Although membrane separation is one of the most generally used methods among these, membranous fine pores of the membrane separation block up easily in removing underwater oils and there is a problem that a membranous usable life becomes short easily. Hence, membrane separation is not appropriate in many cases to remove underwater oils. As a technique for removing heavy oils from oil containing water, the following two methods are recited. For example, one is a method using buoyance of heavy oils. The method allows it to collect heavy oils floating on a water surface with an oil fence and then to vacuum up the oils. The other is a method in which hydrophobic adsorbent is laid on a water surface to collect heavy oils.
As an adsorbing method of underwater oils, there is known a method in which adsorbing polymer with hydrophilic and oleophilic blocks are used to adsorb oils and then the polymer having adsorbed the oils is removed from water. Such a polymer is disclosed by Japanese laid-open patent application JP-A 1995-102238 (Kokai). However, the method has a problem that not only separating an adsorbing polymer from water is troublesome, but also workability of the polymer is low due to softening of the polymer with oils adsorbed.
On the other hand, a method of magnetically separating adsorbent particles having adsorbed oils is also known. For example, Japanese laid-open patent application JP-A 2000-176306 (Kokai) discloses a method in which surfaces of magnetic particles are modified with stearic acid to make the particles adsorb underwater oils for collecting the oil. However, this method also has a problem that low molecular compounds such as stearic acid and a coupling agent adversely contaminate water due to the use of the acid and agent for the surface modifications of the magnetic particles.